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To use the material as a light
emitter, the researchers first had to
convert it into a P-N junction diode,
a device in which one side, the P
side, is positively charged, while the
other, N side, is negatively charged.
In conventional semiconductors,
this is typically done by introducing
chemical impurities into the
material. With the new class of 2-D
materials, however, it can be done
by simply applying a voltage across
metallic gate electrodes placed
side-by-side on top of the material.
“That is a significant breakthrough,
because it means we do not need to
introduce chemical impurities into
the material [to create the diode].
We can do it electrically,” Jarillo-
Herrero says.
Once the diode is produced, the
researchers run a current through
the device, causing it to emit light.
“So by using diodes made of
molybdenum ditelluride, we are
able to fabricate light-emitting
diodes (LEDs) compatible with
silicon chips,” Jarillo-Herrero says.
The device can also be switched
to operate as a photodetector, by
reversing the polarity of the voltage
applied to the device. This causes it
to stop conducting electricity until a
light shines on it, when the current
restarts.
In this way, the devices are able to
both transmit and receive optical
signals.
The device is a proof of concept,
and a great deal of work still needs
to be done before the technology
can be developed into a commercial
product, Jarillo-Herrero says.
This paper fills an important gap in
integrated photonics, by realizing
a high-performance silicon-CMOS-
compatible light source, says Frank
Koppens, a professor of quantum
nano-optoelectronics at the Institute
of Photonic Sciences in Barcelona,
Spain, who was not involved in the
research.
“This work shows that 2-D materials
and Si-CMOS and silicon photonics
are a natural match, and we will
surely see many more applications
coming out of this [area] in the
years to come,” Koppens says.
The
researchers
are
now
investigating other materials that
could be used for on-chip optical
communication.
Most telecommunication systems,
for example, operate using light
with a wavelength of 1.3 or 1.5
micrometers, Jarillo-Herrero says.
However, molybdenum ditelluride
emits light at 1.1 micrometers. This
makes it suitable for use in the
silicon chips found in computers, but
unsuitable for telecommunications
systems.
“It would be highly desirable if we
could develop a similar material,
which could emit and detect light at
1.3or 1.5micrometers inwavelength,
where telecommunication through
optical fiber operates,” he says.
To this end, the researchers are
exploring another ultrathin material
called black phosphorus, which can
be tuned to emit light at different
wavelengths by altering the number
of layers used. They hope to develop
devices with the necessary number
of layers to allow them to emit
light at the two wavelengths while
remaining compatible with silicon.
“The hope is that if we are able to
communicate on-chip via optical
signals instead of electronic signals,
we will be able to do so more quickly,
and while consuming less power,”
Jarillo-Herrero says.
The research was supported by
Center for Excitonics, an EFRC
funded by the U.S. Department of
Energy.
Electro Optic & Camera
Special Edition
image:
Researchers have designed a light-emitter and detector that can be inte-
grated into silicon CMOS chips. This illustration shows a molybdenum ditelluride
light source for silicon photonics.
Credits:
Sampson Wilcox
New-Tech Magazine Europe l 65